Electric arc furnace dust (hereafter referred to as EAF dust) contains elements such as zinc, iron, lead, aluminum, chromium, cadmium, manganese, sodium, potassium, magnesium and calcium. The zinc in the dust is present as zinc oxide and zinc ferrite. The presence of metals such as lead and cadmium in EAF dust make this material a hazardous waste. Disposing of this hazardous waste is expensive and adds to the cost of operating electric arc furnaces. Over the years, attempts have been made to render this byproduct harmless and reclaim some of the constituent elements forming the material in an attempt to lower the costs of disposing of EAF dust. Steel mills using Basic Oxygen Furnace, Blast Furnace and Cupolas generate various iron rich sludges, dusts and mill scales. Recycling these materials economically has become a challenging task to steel mills due to the nature of these secondary materials (referred to as secondary feed stocks).
The recycling of electric arc furnace dusts and various iron bearing secondary materials by pyrometallurgical methods, hydrometallurgical methods and a combination of pyro and hydrometallurgical methods has been the subject of many studies. Pyrometallurgical processes require reducing agents and high temperatures and generally produce a crude zinc oxide of low commercial value. On the other hand hydrometallurgical processes produce high quality metallic zinc or zinc oxide, but most of the processes can not leach zinc completely from zinc ferrite phase unless expensive pressure leaching technology was employed.
The difficulties and drawbacks encountered by several hydrometallurgical technologies developed are discussed. In Canadian patent 1212841 a process for the extraction of zinc from zinc ferrite residues by pressure leaching with sulfuric acid in an autoclave system was disclosed. In Canadian patent 1176853 zinc ferrite was combined with zinc sulfide ore and then pressure treated at elevated temperature to dissolve zinc from the ferrite as well as the sulfide ore. In another Canadian patent 1112880 leaching with aqueous sulfuric acid at an elevated temperature of 140 to 250° C. and 3-40 atm pressure was carried out. These high temperature and high pressure processes are in general are more expensive to the atmospheric leaching processes. In U.S. Pat. No. 4,610,721 atmospherically leaching the steel plant dust in a first stage wherein an amount of steel plant dust is mixed with an amount of acidic zinc sulfate solution to leach zinc there from. Solution pH was controlled between 2.5 and 3.5. Following thickening additional sulfuric acid or the spent electrolyte was added and subjected to pressure leaching. In U.S. Pat. No. 6,338,748 a process was disclosed where hot acid leach containing 37-74 g/L of HCl and 104-270 g/L of ZnCl2 is used. This process claims the dissolution of both zinc oxide and zinc ferrite phases. However, hydrometallurgical processes can not be applied to process other iron bearing materials such as BOF sludge, mill scale and iron ore fines economically as these materials contain zinc values up to about 4%.
Several pyrometallurgical technologies were patented in recent years in US and Canada. Of the various pyrometallurgical studies disclosed, such as U.S. Pat. Nos. 3,770,416; 3,850,613; 4,072,503; 4,396,424; 4,595,574; 4,765,829; 5,013,532; 5,906,671, vaporization of zinc, lead and cadmium from the EAF dust and other zinc containing iron bearing secondary materials was disclosed. In a Canadian and U.S. Pat. Nos. 1,282,965 and 4,800,069, respectively, the recovery of zinc and other metals from strongly bound zinc ferrite compounds are obtained by treating the dust for 1 h at 750° C. with a 20:1 air:chlorine gas mixture whereby zinc, lead and cadmium were removed from the dust as volatilized chlorides. In U.S. Pat. No. 5,906,671 the metals and metal oxides in dust are mixed with a reducing agent and additives, agglomerated, heated above 800° C. and contacted with a flow of inert, reducing or oxidizing gases to volatilize the metals and metal oxides for recovery. In U.S. Pat. No. 4,612,041 and CA 2151195 it was attempted to produce pig iron instead of reduced briquettes or pellets, and it was reported that the pig iron produced contained unacceptable levels of lead. The Waelz process as disclosed in U.S. Pat. No. 4,525,208 consists of mixing the EAF dust with carbon usually in the form of coke or coal and heating the mixture to volatilize zinc. Due to the flow of air during combustion, a substantial amount of iron, calcium, silicon and aluminum compounds are also gas borne within the kiln, and these contaminate the exhaust stream of potentially valuable zinc oxides.
Though the pyrometallurgical processes differed in the techniques such as the type of reactors used, temperatures maintained, amount and type of reducing agent used and in the sequence of oxidation and reduction reactions used, all the processes have resulted in producing a crude zinc oxide product and an intermediate iron product. These processes did not attempt to disclose the method of producing high purity zinc oxide.
In order to overcome the difficulties encountered independently by pyro and hydrometallurgical technologies, inventions using the combination of pyro and hydrometallurgical methods were disclosed: In U.S. Pat. No. 3,676,107 sulfation of the dust, followed by roasting, water leaching was disclosed. However lead was not separated from the iron bearing residue and an additional high temperature operation was needed to remove the small amounts of lead from the iron value, and therefore would not be economical. In U.S. Pat. No. 5,538,532 the EAF dust was heated in the presence of carbon and an additive selected from the group consisting of limestone, silica, calcium chloride and sulfates to a temperature in the range of 1000° C. to 1200° C. and vaporized cadmium, zinc and lead. The condensed vapor dust was slurried in an ammonia-ammonium carbonate solution to dissolve zinc and cadmium. This method still produced an impure zinc oxide that needs to be further refined and does not disclose iron recovery.
In a Canadian patent 2259423 and U.S. Pat. No. 5,942,198 EAF dust is mixed with coal fines to form briquettes. The briquettes are charged to a furnace where zinc is fumed and collected as zinc oxide. The zinc oxide fume thus obtained was leached with ammonium chloride solution at an elevated temperature. The un-dissolved portion of the dust was combined with fresh dust and made into briquettes and charged back to the furnace. In this process the chlorides present in the un-dissolved residue could generate dioxins in the furnace atmosphere due to the addition of coal fines and could cause serious environmental pollution. The zinc values were recovered from intermediate solution phase through water dilution, which means the process requires expensive evaporators to maintain the water balance. In U.S. Pat. No. 6,770,249 EAF dust and other furnace residues were initially fed to the furnace to generate zinc oxide fumes and the zinc oxide fumes were leached in CaCl2 solution and then zinc was recovered as Simonkolleite/zinc-oxychloride/zinc hydroxide product through water dilution.
The paper published in August 1999 Journal of Metals, by I. Palencia et al., on “Recycling EAF Dust Leaching Residue to the Furnace: A Simulation Study”, the recycling of EAF dust leach residue to the furnace was disclosed. The disclosure consists of leaching the EAF Dust with sulfuric acid or with NaOH solution and feeding the un-leached residue containing zinc ferrite to the steel mill furnace. This approach has the disadvantage of introducing un-leached lead and zinc content back to the furnace as sulfuric acid does not leach lead and the caustic solution only partially leaches the lead. This not only increases the lead content of the steel, the zinc present in the un-leached residue would enhance the production of EAF Dust. This disclosure does not include recycling the various iron containing secondary sources.
While the above referenced methods have their advantage, none have proven to be commercially successful, usually due to the costs associated with the methods or due to the inefficient removal of the toxic metals. Therefore a more cost effective method to treat not only the EAF dusts but also the other steel making secondary feed stock residues and sludges is highly desirable. The desired method should enable the economical recovery of zinc and iron values from various secondary feed materials containing zinc and iron values. Further the desired method of recycling iron from such materials should involve minimal contamination caused by metals such as zinc, lead and cadmium.